Modern internal combustion engines may incorporate advanced throttle control systems, such as, for example, intake valve throttle control systems, to improve fuel economy and performance. Generally, intake valve throttle control systems control the flow of gas and air into and out of the engine cylinders by varying the timing and/or lift (i.e., the valve lift profile) of the cylinder valves in response to engine operating parameters, such as engine load, speed, and driver input. For example, the valve lift profile is varied from a relatively high-lift profile under high-load engine operating conditions to a reduced/lower low-lift profile under engine operating conditions of moderate and low loads.
Intake valve throttle control systems vary the valve lift profile through the use of various mechanical and/or electromechanical configurations, collectively referred to herein as variable valve actuation (VVA) mechanisms. Several examples of particular VVA mechanisms are detailed in commonly-assigned U.S. Pat. No. 5,937,809, the disclosure of which is incorporated herein by reference. Generally, a conventional VVA mechanism includes a rocker arm that carries a cam follower. The cam follower engages an input cam of a rotary input shaft, such as the engine camshaft. The cam follower and thus the rocker arm are displaced in a generally radial direction by the input cam, and a pair of link arms transfers the displacement of the rocker arm to pivotal oscillation of a pair of output cams relative to the input shaft or camshaft. Each of the output cams is associated with a respective valve. The pivotal oscillation of the output cams is transferred to actuation of the valves by associated cam followers, such as, for example, direct acting cam followers or roller finger followers. One or more return springs biases the rocker arm cam follower into engagement with the input cam lobe.
A desired valve lift profile is obtained by orienting the output cams in a starting or base angular orientation relative to the cam followers and/or the central axis of the input shaft. The starting or base angular orientation of the output cams determines the portion of the lift profile thereof that engages the cam followers as the output cams are pivotally oscillated, and thereby determines the valve lift profile. The starting or base angular orientation of the output cams is set via a control shaft that pivots a pair of frame members which, via the rocker arm and link arms, pivot the output cams to the desired base angular orientation.
Typically, the frame members and output cams of a conventional VVA mechanism are pivotally disposed upon the engine input or camshaft. Thus disposed, the frame members and output cams impose parasitic loads upon the driving torque of the engine input/camshaft. Such parasitic loads reduce engine power and fuel efficiency. Further, since the rocker arm is connected via the link arms to the output cams, the return spring must provide sufficient force to overcome the inertia presented by these components in order to maintain the rocker arm cam follower in contact with the input cam lobe, and must be stiff enough to do so at relatively high engine-operating speeds. The design of a spring having sufficient force and stiffness, and yet small enough to fit within the limited space available in a modern engine, requires complex spring designs and relatively expensive materials. Moreover, the relatively large number of component parts and critical interfaces within a conventional VVA mechanism make their manufacture and assembly relatively complex, labor intensive and costly.
Therefore, what is needed in the art is a VVA mechanism that has fewer component parts and is therefore easier to manufacture and assemble.
Furthermore, what is needed in the art is a VVA mechanism that places little or no parasitic load upon the driving torque of the engine input/camshaft, and thereby improves engine power and fuel efficiency.
Moreover, what is needed in the art is a VVA mechanism that reduces the stiffness required of the return spring by reducing the effective mass of the components of the VVA, thereby enabling an increase in the maximum engine operating speed at which the VVA can be used.